The present invention relates to an apparatus for chopping a charged particle beam and, more particularly, to an apparatus for chopping an electron beam which is suitable for use in a stroboscopic scanning electron microscope.
Stroboscopic scanning electron microscopes are constituted by an ordinary scanning electron microscope and a beam chopper and synchronous circuit added thereto. This microscope will be described hereinbelow with reference to the drawings. FIG. 1 is a diagram showing a fundamental arrangement of a stroboscopic scanning electron microscope. An electron beam 2 emitted from an electron gun 1 is focused on a sample 10 using an electron lens 6 and is caused to scan rectangularly by a scanning coil 8 in the same manner as the scanning is performed in an image pickup tube of a television set. When the electron beam collides with a solid, a secondary electron or a reflection electron is emitted therefrom. This secondary electron or reflection electron is detected by a detector 9 and in response to the intensity of the detected signal, an image of the sample is displayed on a display device 7 which operates synchronously with the scanning coil 8. This is a fundamental principle of the scanning electron microscope.
However, in the case where a sample (for example, an integrated circuit IC or the like) which is changing at high speeds is observed by this scanning electron microscope, the scanning velocity cannot follow the changing speed of the sample, so that the whole change of the sample will have been overlapped and displayed. In view of the above situation, the electron beam is controlled so that it is deflected by imparting to a deflecting plate 3 an output of a pulse circuit 12 synchronized with a driver 11 which causes the change in the sample change (for instance, in position such as A, B and C in FIG. 1). The electron beam passes downwardly through an aperture 4 only when it is deflected on an opening of the aperture 4, and thus the electron beam is chopped here. By scanning the sample with such a chopped electron beam, the electron beam is irradiated only at a constant phase point in the sample change; consequently, only an image at a certain time point (phase) in the sample condition can be selectively displayed without causing the overlap of the whole movement. This is the stroboscopic scanning electron microscope (refer to G. S. Plows and W. C. Nixon; "Stroboscopic Scanning Electron Microscopy", J. Phys. E., Ser. II, pp 595-600, 1968).
A delay circuit 5 is provided between the pulse oscillator 12 and the driver 11 in order to observe the whole sample change by use of the above-mentioned method. FIG. 2 shows a circuit for generating a delay which is most generally used. A trigger pulse from the driver 11 is inputted to a terminal 16. In response to this trigger pulse, a saw-tooth oscillator 13 generates a saw-tooth wave synchronized with the driver 11. This saw-tooth wave is inputted to one input of a phase shifter 15. An output from a DC power source 14 is inputted to the other input. An output of the DC power source 14 is controlled manually or by a computer. The phase shifter 15 compares those two inputs and generates a pulse when their levels coincide. FIGS. 3A to 3C show the time relation between those signals. In FIG. 3A, a denotes a trigger pulse which is inputted to the saw-tooth oscillator 13. In FIG. 3B, b indicates an output of the saw-tooth oscillator 13 and c represents an output of the DC power source 14. In FIG. 3C, d is an output from the phase shifter 15. A time delay of td is caused between a and d. This td can be set to an arbitrary value by changing a level of c.
However, the minimum time .DELTA.t which can be controlled by the phase shifter 15 is determined by a noise .DELTA.Vs of the saw-tooth wave, a noise .DELTA.Vd of the DC voltage and a jitter .DELTA.tp of the phase shifter. Assuming that the rise time of the saw-tooth wave is k(ns/V), .DELTA.t will become ##EQU1## .DELTA.Vd can be ignored since it is smaller than .DELTA.Vs. When it is assumed that .DELTA.Vs is 10 mV and k is 0.2, k.multidot..DELTA.Vs will become 20 ps. Although .DELTA.tp differs depending upon components used in the phase shifter, it is at least 20 to 30 ps and, as a whole, .DELTA.t can not be made shorter than about 50 ps.
Although the limitation imposed on the phase shifter has been described, it is said that about 50 ps is the limit when a delay line is used. This is due to a difficulty in adjusting the delay line which has a length on the order of millimeters. A variation in length of terminals of a sample, for example, an IC is at least over three millimeters, so that this causes a variation in delay amount of 20 ps.